The present invention relates to the field of sound generation. More particularly, the invention relates to a device for producing sound waves in a body of water.
There are many circumstances in which it is desirable to produce sound waves in a body of water. Seismic visualization of marine oil reservoirs is one example. Seismic visualization is a technique used to determine the size and shape of underground oil reservoirs. The technique involves radiating sound waves into the earth's surface above the reservoir and then capturing the reflected sound waves. The acoustic characteristics of the reflected sound waves are then analyzed to visualize the size and shape of the underground reservoir as well as the geological formations in the area surrounding the reservoir. This technique is useful when exploring for new oil reservoirs and also for managing oil production from a known oil reservoir.
Exploring for new oil reservoirs typically involves methodically visualizing a particular subterranean area. A basic level of visual detail is needed to determine if and where an oil reservoir is located. However, after an oil reservoir has been located, a more detailed view of the reservoir and surrounding areas is required to effectively manage the production of oil from the reservoir. The increased visual detail is required to locate optimal drilling locations to maximize the production of oil.
The level of visual detail afforded by a seismic visualization system is largely dependent upon the capabilities of the sound generating device. In general, a device producing a large bandwidth of frequencies can provide the more detailed visualization. Typically, when exploring for new oil reservoirs, devices producing frequencies of less than 70 Hz are capable of generating a resolution detailed enough to locate oil reservoirs. In production management, however, greater frequencies are required to adequately visualize the geological features surrounding the oil reservoir.
Generating sound waves in a body of water for seismic visualization purposes is typically performed with a device called an air gun array. The air guns within the array are detonated above the sea bottom to generate a sound wave that travels through the water to the earth's surface. The sound is partially transmitted through the surface and is partially reflected back from the stratified geological interfaces below. The reflections are sensed by any of several any of several known devices that are capable of interpreting the echoes and producing the visualization of the subterranean area.
The use of air guns to seismically visualize oil reservoirs in a marine environment presents several problems. Because of the design of the typical air gun, a large air gun is required to generate a sound wave having enough power to effectively visualize a reservoir. Thus, a large carrying vessel is required. In addition, the sound generated by the air gun cannot be radiated preferentially downward towards the oil reservoir. As a result a large amount of unwanted signals are echoed back from undesirable objects. A significant amount of signal processing is required to filter out the undesired echoes.
The frequency bandwidth of air guns is also limited. Currently, known air guns radiate insufficient power at frequencies greater than about 70 Hz. The limited frequency bandwidth provides insufficient resolution to visualize the oil--water interface within the formation. It is of great importance to effective oil - field management that successive locations of this interface be visualized at different times during production. In addition, the great power output of the air guns directed into a water body presents several environmental concerns. In particular, air guns are believed to present a serious threat to the nearby sea life.
In light of the foregoing there is a need for an environmentally friendly device capable of radiating a low frequency, high powered, and broad band of sound.